Capacitor bank on low voltage side of the transformer 1

[electpower]

Hello,

I heard from my collegue that a capacitor bank has to be installed in the low voltage side of the generator step up transformer, which is connected to the generator.
The high voltage side of the generator step up transformer is connected to the network (grid). He also mentioned that a short circuit in the primary side of the tranformer with secondary side open , will give an overvoltage value in the secondary side. The capacitor bank has to be designed in accordance to the short circuit current , in order to avoid the overvoltage.

Could some one explain me the phenomena behind this, and please suggest me with some literature to get some knowledge on this, or some international standards.

Regards
Electpower

 

[electpower]

The primary is connected to the hv side and the secondary to the low voltage side

 

[waross]

electpower;
Actually, primary and secondary have nothing to do with voltage levels. The primary is the side that is connected to the source and the secondary is connected to the load.
In a Generator Stepup Transformer, the primary is, by definition, the lower voltage side.
Capacitors on the high side reduces the transformer current and greatly reduces the size of the capacitor bank. That is the physical size and the capacitive reactance rating, not the kVAR rating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

electpower,
The capacitor that you are referring is a surge absorber.It is connected to protect the generator winding from the transfered surges through the GT.When a surge approach from HV side,a considearcble part is transferred to LV side by capacitive and magnetic coupling.Many times this will be more than the BIL of transformer LV winding /generator stator winding.The C connected across the LV terminals reduces the magnitude of transferred surge from HV.This is the reason,normally this capacitor is insisted, when back charging the transfomer from HV side without the generator connected.

 

[RRaghunath]

Agree with you, prc. It is for smoothening of transferred voltage surges from HV / grid side.
It is normally a combination of capacitor and the surge arrester at generator terminals.
It is relevant if the grid comprises OHL network and is required irrespective of whether back charging is envisaged or not.

 

[prc]

Not exactly Raghun.Transfered surge on LV is different with generator connected and without connected.When generator is connected it comes down.So when back charging(ie generator will not be ready, but want to charge trf)the surge arrester of GT shall be put on the trf LV terminals.

 

[RRaghunath]

prc,

Transformer BIL is better than that of Generator. It is by design. Hence, when generator is not connected the requirement for surge suppression is not so much.
To illustrate the point, we never think of connecting surge suppression module for a step-down transformer secondary (hope you agree) when it is supplying just a load switchboard.

 

[cuky2000]

The capacitor on the LV side of the GSU will reduce the slope of the incoming overvoltage wave protecting the turn-to-turn genenerator winding. Usualy, surge arresters are used in combination with the surge capacitor to reduce the peak overvoltage above a protective treshole.
Overvoltage under light load or no load in generator with surge capacitor it is similar to the capacitance effect in long line better known as Ferrantti effect.

 

[prc]

Raghun,
Dont compare a high voltage ratio GT with stepdown transformer.The magnetically transfered component will be different.Similarly transfered surge in to a single phase LV and delta Lv are different (later case much less).If the GT is designed for full transferred surge (that will be more than the nominal BIL of LV) it is OK to back charge with out Capacitor.Better to consult the manufcaturer who invariably will be suggesting to put the LA &C on LV terminals.

 

[PiotrM]

Hi all,

do you have any expierence with maintenance of power transformer with capacitor bank in the tertiary side? I'm not asking about GSU but 160MVA and higher power transformers.

We have a few such instalations but I am worring about these units. I have heard about problems with charging the capacitor bank on and off.

PiotrM